A conductive paste is a paste that contains a metal micropowder as the main conductive material in a binder resin such as polyamide-imide, poly(ether ether ketone), epoxy resin, phenolic resin or the like, that is used as an raw material for electrically-conducting path, for example when formed into the electrically-conducting circuit of a printed wiring board. Moreover, in double-sided printed wiring boards and build-up multilayer printed wiring boards, through-holes are filled with a conductive silver paste to connect the wiring patterns of each layer. It is also known that a conductive paste can be used to form a thick film conductor circuit on a ceramic substrate.
For example, a conductive ink is proposed in U.S. Pat. No. 5,882,722, where a metal powder and a metallo-organic decomposition (MOD) compound are dispersed in an organic liquid vehicle. This conductive ink can be printed onto a printed wiring board or flexible printed wiring board using screen printing, gravure printing or the like. When an MOD compound like silver neodecanoate is heated to a temperature on the order of 200 degrees C., decomposition begins and the metal precipitates. Since the precipitated metal has a small particle size, it is highly activated, and furthermore can undergo sintering at relatively low temperatures. When this conductive ink is printed onto the substrate and is fired at a temperature less than 450 degrees C., the metal precipitated from the MOD compound promotes bonding within the metal powder and bonding to the substrate to form the conductive metallic coating.
Nevertheless, since this type of conductive ink is constituted substantially from silver alone, the conductive coating obtained has inadequate flexibility, and with a thick film the bending resistance will be lowered.
In order to improve the inadequate flexibility of this conductive coating, for example, in Japanese Published Unexamined Patent Application No. 2004-039379, a silver-containing organic compound is used that possesses a constitution wherein the silver atoms and the organic component are bonded through a heteroatom, but the layer thickness exhibited in the working example is a maximum of 25 μm, and there is the additional problem that the silver-containing organic compound generates hydrogen carbide and carbon dioxide gas during firing.
Additionally, as described in the present comparative examples, when a silver-containing conductive paste is manufactured using a polyimide precursor used as the binder resin that is polymerized from an aromatic tetracarboxylic acid dianhydride and an aromatic diamine in a polar organic solvent this type of conductive paste becomes a flocculent type powder as the imidization progresses, and there is a problem in that no coating is obtained, let alone any film.